The present invention relates to internal combustion engines in general, especially to diesel engines. More particularly, the invention relates to improvements in piston drives which transmit motion to the crankshafts of internal cumbustion engines. Still more particularly, the invention relates to improvements in piston drives of the type wherein the piston comprises two sections which are axially spaced apart from one another and one of which guides the other.
The other section carries one or more sealing rings which slide along the internal surface of the cylinder to seal the combustion chamber from the interior of the cylinder. The piston is coupled to a connecting rod which is connected to a crank pin of and transmits torque to the crankshaft in response to repeated combustion of a fuel-air mixture in the combustion chamber. A pivot member is provided to movably couple the two sections of the piston and the respective end portion of the connecting rod to each other.
In piston drives of the above outlined character, the sealing section of the piston comprises a crown and a carrier portion which latter mounts several sealing rings and has an external surface adjacent to the cylindrical internal surface of the cylinder. The sealing section of the piston further comprises a supporting portion which extends from the crown to the base portion of the piston and expands or diverges radially in a direction from the base portion toward the crown. It is also known to provide the sealing section of the piston with a recess or groove which is disposed between the crown and the carrier portion and is adjacent to the outermost part of the supporting portion of the sealing section. The two sections of the piston have mirror symmetrical cylindrical internal surfaces which surround the pivot member, and a further cylindrical internal surface is provided on the respective end portion of the connecting rod to surround a part of the pivot member and to thus enable the connecting rod to drive the crankshaft in response to reciprocatory movements of the piston in the cylinder chamber.
Piston drives of the above outlined character are used in a great majority of internal combustion engines, especially diesel engines, to transmit motion between the piston or pistons and the crankshaft in response to combustion of fuel in the combustion chamber or chambers. Each piston moves axially of the respective cylinder in one direction in response to expansion pressure which develops as a result of combustion of the fuel-air mixture, and the piston moves in the opposite direction in response to continuous rotation of the crankshaft. The combustion progresses in any one of a number of known ways as a result of direct or indirect injection of fuel into the combustion chamber. In presently known diesel engines, the fuel jet or a jet of fuel-air mixture which is ignited before it reaches the interior of the cylinder is directed against the wall which surrounds the combustion chamber in the piston or against the crown of the piston. If the piston is made of one or more materials which are good conductors of heat, such as aluminum, there often develop localized hot cracks or, at the very least, pronounced heat losses of the working gas, especially if the material of the piston transmits substantial amounts of heat to the cylinder and thence into the coolant of the internal combustion engine. Such piston drives are far from being satisfactory, and attempts to avoid excessive heating of the cylinder, especially in engines employing piston drives which are expected to be subjected to very pronounced stresses, include the utilization of piston drives with built up pistons which consist of steel and aluminum. The cost of such built up pistons is very high so that the piston drives employing pistons which consist of several metallic materials are used only under special circumstances. The just discussed built up pistons can be subjected to more pronounced stresses in view of the utilization of steel, and their weight is relatively low due to the utilization of aluminum. Nevertheless, there exists an urgent need for piston drives, especially for use in passenger car engines, whose weight is low or relatively low, whose cost is much less than that of piston drives using the aforediscussed built up pistons, which can stand pronounced or very pronounced stresses, and whose heat insulating properties are satisfactory in spite of the fact that the piston drives satisfy the just mentioned requirements regarding the weight, cost and resistance to stresses.
In accordance with a presently known proposal, a piston drive which is supposed to exhibit satisfactory characteristics regarding the resistance to stresses and heat insulation on the one hand, as well as relatively low weight on the other hand, employs a two-piece piston having a guiding and a sealing section, and a pivot member which couples the two sections of the piston to the connecting rod. The material of the sealing section exhibits satisfactory heat insulating properties, and the material of the guiding section is selected with a view to ensure that this section can readily slide in the cylinder of the internal combustion engine. The two sections are separated from one another by a clearance and the sealing section is formed with circumferentially complete grooves for a set of sealing rings which slide along the internal surface of the cylinder. As already mentioned above, the sealing section of the piston further comprises a supporting portion which connects the crown and the carrier portion for sealing rings with the base portion and is adjacent to the inner side of the aforementioned annular recess between the crown and the carrier. The conically outwardly diverging part of the supporting portion can surround a combustion chamber if such chamber is provided in the crown of the sealing section; the recess then constitutes a heat barrier between the combustion chamber and the carrier portion for sealing rings to prevent the transfer of substantial quantities of heat to the cylinder and thence to the cooling medium. The base portion of the piston is bifurcated, and the prongs of such base portion are mirror symmetrical to each other with reference to the axis of the cylinder. These prongs are formed with bearing portions for the pivot member. The guiding section of the piston also comprises bearing portions which surround the pivot member so that the latter couples the two sections of the piston to one another. The respective end portion of the connecting rod is provided with one or more additional bearing portions for the pivot member. In order to remove heat which develops as a result of combustion of fuel and is transmitted from the combustion chamber to the base portion of the piston, as well as to ensure adequate lubrication of the joint between the connecting rod and the two sections of the piston, the conventional piston drive further comprises a guide face which is provided at the inner side of the guiding section and is oriented and configurated in such a way that a cooling and/or lubricating medium which is sprayed against the guide face is caused to flow toward the joint, i.e., toward the pivot member in the region of the base portion of the piston. It has been found that the just discussed previously known piston drive is capable of standing pronounced stresses and also that it exhibits satisfactory heat insulating properties. Nevertheless, when the stresses are highly pronounced, it is necessary to undertake additional measures in order to prevent the application of excessive bending and/or shearing stresses to the pivot member which couples the connecting rod with the two sections of the piston. Shielding of the pivot member against excessive bending and/or shearing stresses is desirable and necessary when the pivot member is to stand surface pressures in the range of up to and even in excess of 2000 kg/cm.sup.2. Reference may be had to German Offenlegungsschrift No. 25 43 478.
In accordance with a further prior proposal, and in order to ensure that the pivot member will stand highly pronounced bending and/or other stresses, the two-piece piston (which again includes a guiding and a sealing section) is constructed and assembled in such a way that the external surface of the conical supporting portion of the sealing section of the piston has a base in the region of the crown, and such base merges into the adjacent surface of the crown in the region of a so-called gravity circle whose diameter equals or closely approximates seventy percent of the piston diameter. Such connection between the base of the external surface of the conical supporting portion of the sealing section and the crown is intended to prevent bending or flexing of the crown and to ensure that the forces which develop on combustion of fuel are transmitted to the pivot member without the development of bending moments. This piston drive has found widespread acceptance in the relevant industries; however, it cannot invariably prevent the development of problems in connection with bracing of the pivot member on the connecting rod, especially when the piston drive is subjected to pronounced or highly pronounced stresses. The demands upon the stability of the piston drives are continuously on the increase, especially as concerns the static behavior of the piston drives, because such drives must stand elevated pressures in the range and even in excess of 200 bar as well as surface pressures in the range of up to and in excess of 2000 kg/cm.sup.2. Under such extreme operating conditions, the pivot member is likely to undergo deformation which can entail damage to bearing portions, breakage of the pivot member and/or breakage of the guiding section of the piston. Reference may be had to German Offenlegungsschrift No. 27 17 028.